1. Lightly grease the slow cooker with butter.
2. Arrange a third of the potato slices in the bottom of the slow cooker. Top with a third of the finely chopped onion and a third of the crumbled bacon.
3. In a medium saucepan, melt the unsalted butter over medium heat. Stir in the flour until smooth.
4. Gradually whisk in the heavy cream and milk, bringing the mixture to a simmer.
5. Add the minced garlic, salt, and black pepper, continuing to cook until the sauce thickens, about 5 minutes.
6. Remove the saucepan from heat and stir in 1 1/2 cups of the shredded cheddar cheese, and 1/2 cup of the shredded mozzarella cheese until melted and smooth.
7. Pour a third of the cheese sauce over the potatoes in the slow cooker.
8. Repeat the layers twice more: potatoes, onion, bacon, and cheese sauce.
9. Sprinkle the remaining shredded cheese over the top.
10. Cover and cook on low for 6-7 hours or on high for 3-4 hours, until the potatoes are tender and the cheese is bubbly.
11. Garnish with chopped fresh parsley before serving, if desired.
